Environmental Engineering Reference
In-Depth Information
extinction of the local population, vacates that space. The assumption is that there is
a trade-off between competitive ability and colonizing ability, and this means that
vacant space is likely to be taken by an inferior competitor. Ecological succession
takes place as inferior competitors are replaced by later arriving superior competitors.
Because local sites are vacated at different times, and recolonization and succession
have stochastic timing, landscapes governed by these process will consist of a mosaic
of sites in different successional stages. For this reason, this hypothesis is also known
as the successional mosaic hypothesis [
35
]. It is one version of the intermediate
disturbance hypothesis [
78
-
80
], and perhaps the version closest to the original inten-
tion of the idea [
79
].
In these discussions, disturbance is a natural process such as fire or extreme
weather that destroys local populations patchily in space. In one version, predators
are the agents that destroy local populations [
37
,
81
], and thus maintain a patchy
landscape in a mosaic of successional states. The competition-colonization trade-
off hypothesis, however, can work without an agent of disturbance, but relying on
chance mortality of individuals, dispersal and colonization [
82
]. This successional
mosiac process can be modeled well by Lotka-Volterra competition equations with
density measured at the landscape scale as the fraction of sites occupied by
a species [
76
,
77
,
83
]. Its form is the same as an asymmetric interference competi-
tion model, but nevertheless governed by the coexistence conditions (
Eq. 13.3
)
above, which mean that all species inhibit themselves more than they do other
species. For superior competitors, this outcome occurs because inferior competitors
are better at finding free species and so escape interspecific competition from
superior competitors. This idea is also related to nonspatial models of exploitation
of leftover resources, for example light not intercepted by a plant canopy, and so
available to understorey species [
84
,
85
]. Fundamentally, in the competition-colo-
nization trade-off hypothesis and leftover resource models, superior competitors do
not efficiently exploit all resources, leaving some to be exploited by those species
that have lesser competitive ability but through a trade-off have achieved the ability
to exploit the leftovers.
A final spatial mechanism involves natural enemies. Known as the Janzen-
Connell hypothesis [
86
-
88
], it was originally proposed for tropical trees, but is
closely related to the soil-feedback hypothesis for coexistence of species in
grasslands [
89
]. The idea as applied to trees was that natural enemies specialized
on a particular species would build up in abundance on or near a given tree. These
natural enemies would then provide strong inhibition to the establishment of
individuals of the same species there. Other species, however, would be able to
establish. It is clear, however, that this is a form of natural enemy partitioning that
does not require a spatial element, although it might well be enhanced by the spatial
element. In the soil-feedback hypothesis, soil microorganism communities develop
in the root zone of a particular individual plant. A preponderance of relatively
species-specific harmful microorganisms leads to a net negative effect of establish-
ment of the same species at that site, favoring others species to replace that individual
or to thrive nearby [
89
,
90
].
